1. Field of the Invention
The invention in general relates to fluid detection systems, such as for detecting the presence of or the amount of gasoline in or outside of a tank, and in particular, such a detection system having an overfill probe.
2. Description of the Prior Art
U.S. Pat. No. 4,646,069 issued to Raymond J. Andrejasich and Roy E. Kidd discloses a fluid detection system which is exemplary of systems for detecting the presence of fluid pollutants. Such systems typically include detection probes which are placed in the region of possible sources of pollutants, such as gasoline storage tanks. The probes are connected by wires to a central monitoring station which monitors the probe status. In many such systems, the monitor provides a plurality of probe signals, each signal associated with a specific probe. Further, each probe includes a receiving and sending circuit which includes an identifier number assigned to the probe. The probe provides a signal indicating the probe's status upon reception of the probe signal associated with the identifier number assigned to that probe In this way, the monitor can provide the probe status correlated with specific probes.
Pollulert.TM. Fluid Detection Systems Installation and Operation Instructions for Pollulert Control Center FD-103, Pollulert Systems document No. 961-40355-01 Revision Level 3, particularly at pages 24 through 27, describes a system such as that described in the above-mentioned U.S. patent that also includes an overfill probe. In this system, the overfill probe is assigned an identifier number and reports its status just as any other probe
Generally, leak detection systems for tanks are designed to check the status of the probe about once a minute. This is more than sufficient since leaks generally develop in a tank over relatively long periods or are caused by catastrophic events which draw attention to the leak. However, it is generally preferable to check an overfill probe on the order of every five seconds or so, since otherwise one would have to set the threshold level such that the approaching overfill is alarmed at a level which would leave a relatively large volume of the tank unfilled at the time of alarm. This presents a problem since checking all the probes in a system such as described above at a sufficiently fast rate that the overfill probe is checked every five seconds or so would require much more expensive electronics. U.S. Pat. No. 4,835,522 describes a fluid detection system which solves this problem by checking the overfill probe in a special routine that is implemented only when a flow meter indicates that the tank is being filled. While this is effective, it does require a flow meter, which is relatively expensive. It would be useful to have a leak detection system in which an overfill probe could be included without also requiring the more expensive electronics or a flow meter.